Sulfur/carbon ratios and extractable organic matter of the middle proterozoic nonesuch formation, north american midcontinent rift

Abstract

The Nonesuch Formation (ca. 1.1 Ga), host to the White Pine copper deposit, is a generally organic-rich unit of relatively wide extent. Selected strata, away from mineralization, were analyzed for their organic carbon and total sulfur contents and for the amount and composition of extractable organic matter. Sulfur to carbon ratios are covariant, but the ratios of total sulfur to organic carbon are generally higher than those found for marine Phanerozoic black shales deposited under oxic bottom water conditions. The sulfur to carbon ratios and the intimate association fo framboidal pyrite and organic matter are suggested to be the result of bacterial sulfate reduction in the Nonesuch sediments. Activity of sulfate reducing bacteria requires a supply of sulfate in the water column and implies that the depositional environment was not a low-sulfate aqueous environment. A marine embayment dominated more by sea water than riverine input is suggested as the environment of deposition based on the correlation of total sulfur and organic carbon, the relatively high total sulfur content, and on the absence of sedimentologic and geochemical criteria for saline lacustrine environments. Deposition of highly reactive organic matter in low concentrations coupled with high concentrations of reactive iron and the absence of bioturbation resulted in unusually efficient pyrite formation. Extractable organic matter in unmineralized Nonesuch strata is interpreted to be autochthonous in origin. Homologous series of n-alkanes and cyclohexyl alkanes, regular isoprenoids and monomethyl-branched alkanes were identified by gas chromatography. Ratios of pristane/phytane, pristane/n-C17 and phytane/n-C18 suggest relatively low thermal maturity for the overall unit with a slight increase in the level of thermal maturity from Michigan westward to Wisconsin. The aliphatic character and high extractability of organic matter indicate that the petroleum potential of the Nonesuch Formation will be good to excellent if the organic-rich facies thicken down-dip under Lake Superior or into the southern extension of the rift.